Epoxy resins have been employed in a wide field such as adhesion, casting, encapsulating, laminating, molding, coating, etc. Because of the excellent physical properties after curing and ease in handling. Further, epoxy resin includes many kinds, and since physical properties after curing largely vary according to the selection, it has been separately employed according to use fields and purposes.
In recent years, with a progress of severe conditions in uses of polymeric materials, a variety of properties required for the polymeric materials are becoming severer, and requirements in properties have not become able to be sufficiently satisfied in various epoxy resins which are usually employed.
For example, although epoxy resin compositions are employed for encapsulating semiconductors, requirements in properties are getting severer even in the field. That is, semiconductor devices become highly integrated owing to semiconductor chips having a remarkably large capacity, and packages themselves become small-sized and thin-sized. Still further, in the semiconductor devices, mounting also shifts to a surface-mounting, and the semiconductor devices in the surface mounting are directly immersed into a solder bath. Accordingly, those are exposed to a high temperature circumstance, and a large stress is wholly induced in the package because of abrupt expansion by absorbed moisture, and cracks are caused in encapsulants. For that reason, for the epoxy resin composition for encapsulants having excellent solder crack resistance, there are required a high heat resistance (that is, high glass transition temperatures), a low moisture absorption property, and a low stress property.
By mixing a large amount of inorganic fillers such as fused silica powder, there has been widely carried out to improve a low moisture absorption property and a low stress property (that is, a low thermal expansion ratio), and it is largely effective for an improvement of the solder crack resistance. However, since mixing a large amount of inorganic fillers deteriorates fluidity in molding, low melt viscosity has been also required in the epoxy resin composition for encapsulants.
Moreover, high fluidity has been also required in the epoxy resin composition for encapsulating semiconductors with small-sizing and thin-sizing in the package, and lowering in melt viscosity has become more strictly required for the epoxy resin.
Cresol-novolak type epoxy resins which are mainly employed at present time have not become regarded as sufficient in view of a low moisture absorption property and low melt viscosity.
In a process for the preparation of the epoxy resin composition for encapsulating semiconductors, there is usually conducted a process in which respective materials are melt kneaded after crushing and, further crushed while cooling.
Accordingly, raw materials require capability of being respectively crushed. That is, an epoxy resin having a softening point of not more than 55.degree. C. is too soft and sticky to crush at an ordinary temperature. Therefore, it cannot be employed, or there are required specified apparatuses and conditions. In general, an epoxy resin having a low molecular weight has low melt viscosity and, in the case of a usual noncrystalline epoxy resin, a softening point lowers with lowering of a molecular weight, and since it becomes semi-solid state or liquid, it cannot be crushed at an ordinary temperature.
Accordingly, even in an epoxy resin having excellent physical properties as an epoxy resin composition for encapsulating semiconductors such as the cresol-novolak type epoxy resins, lowering in melt viscosity is limited because of incapability of lowering a molecular weight in which a softening point becomes not more than 55.degree. C., resulting in that the solder crack resistance cannot be sufficiently improved. Although it is investigated (Japanese Patent Application Laid-open No. Sho 61-047725, etc.) that there is employed a tetramethylbiphenol type epoxy resin having low melt viscosity because of a low molecular weight, and which can be crushed because of becoming crystalline at an ordinary temperature, solder crack resistance cannot be sufficiently improved because of a poor heat resistance in a cured product therefrom. Further, although a 4,4'-biphenol type epoxy resin (Japanese Patent Application Laid-open No. Hei 1-230619), which also becomes crystalline, is low in melt viscosity and also excellent in heat resistance, it cannot be solely employed because of a high melting point and also poor compatibility with a curing agent, etc. There is a problem that melt viscosity increases in a resin system when mixed with a conventional noncrystalline solid epoxy resin.